This invention pertains to high solids acrylic coating systems and more particularly to the use of low molecular weight hydroxyl-containing acrylic polymers as reactive diluents.
The U.S. Environmental Protection Agency has lowered the allowable solvent emission limits for the coatings industry and intends to make them more stringent in the future. Several ways to lower the solvent emissions from coatings is to recover the solvent in the emissions or to incinerate the emissions. Other alternatives are waterborne coatings where the emission of water is, of course, allowable or high solids coatings which contain much less solvent than conventional coatings and may meet the emission guidelines. The recovery or incineration of solvent emissions requires a great investment of capital in equipment and waterborne coatings, while applicable in certain areas, do not possess good outdoor weatherability and are deficient in other properties. According to the above facts, the only viable high performance system is high solids coatings.
There are several ways to prepare high solids coatings systems. By reducing the molecular weight of the polymer that comprises the coating, the viscosity for a given solids content can be lowered. Also, a system where the solvent does not evaporate but reacts with the resin system to become part of the coating can be used. This is usually termed a reactive diluent. In the former case, lowering the molecular weight of the polymer causes a deterioration in the properties of the coating. Therefore, the best approach to high solids coating is the reactive diluent.
Acrylic resins are very common and widely used in the coatings industry and are almost exclusively used by one of the major consumers of coatings, the automotive industry. Also, one of the most common crosslinking reactions for acrylic polymers used in coatings is the reaction of hydroxyl groups on the polymer with crosslinking agents such as formaldehyde-melamine condensates and isocyanates.